The role of friction on sharp indentation

Frictional effects on sharp indentation of strain hardening solids are examined in this paper. The results of finite element simulations in a wide range of solids allow us to derive two simplified equations, accounting for the influence of the friction coefficient on hardness. Comparisons between the simulations and instrumented micro-indentation experiments are undertaken to ensure the validity of the former to metallic materials. Quantitative estimates of the role of friction on the development of pileup and sinking-in around the contact boundary are also given in the paper. These results provide a physical insight into the plastic flow features of distinctly different solids brought into contact with sharp indenters. Overall, the investigation shows that the amount of pileup can be used to set the range of validity of the two hardness equations indicated above. Friction has the largest influence on the contact response of solids exhibiting considerable piling-up effects (whose parameter , see text for details), whereas materials developing moderate pileup or sinking-in are less sensitive to friction. Finally, a methodology is devised to assess the influence of the friction coefficient on mechanical properties extracted through indentation experiments.     

Characterisation of roof tile coating degradation using nano-indentation test and surface profilometry

Nano-science and nano-technology have been extensively explored to develop functional and smart coating systems. In this context, it is important to investigate novel characterisation methodologies for surface feature measurement and evaluation of diverse coating systems at nano and micrometrical scales which cannot be implemented by conventional tests. In this study, nano-indentation test have been carried out to characterise the environmental degradation of a polymer coating on a building material substrate. Multiple locations were selected for nano-indention for analysis and evaluation. It has shown that the coating weathering led to a reduction in deformation depth and an increase of hardness and modulus. Furthermore, the weathering resulted in surface quality degradation, which was characterised by hugely reduced surface homogeneity in terms of standard deviations in deformation, hardness and modulus. The coating degradations were also characterised by surface roughness measurement by profilometry. There were significant differences in surface roughness at the micrometrical scale for coatings with varying environmental degradations. They were quantified in terms of surface roughness statistics. The micro morphologies of the coating were observed by SEM which showed a good agreement with results from nano-indentation test and surface roughness measurement. This study demonstrates that nano-indentation test and surface roughness measurement are viable methodologies to evaluate specialised polymer coating systems and to characterise their degradations at the nano and micrometrical scales.     

Molecular dynamics simulation of nano-indentation on Ti-V multilayered thin films

We developed a second nearest-neighbor modified embedded-atom method potential for binary Ti-V system. The potential parameters were identified by fitting the lattice parameter, cohesive energy and elastic constants of CsCl-type TiV, and further validated by reproducing the fundamental physical and mechanical properties of Ti-V systems with other crystal structures. In addition, we also performed molecular dynamics simulations of nano-indentation processes of pure Ti film, pure V film, and two kinds of four-layer Ti-V films, V-Ti-V-Ti and Ti-V-Ti-V. We found that the indentation force-depth curve for the pure V film turns flat at an indentation depth of 2.8 nm, where a prismatic loop was observed. Such prismatic loop is not found in the V/Ti/V/Ti multilayer because the thickness of each layer is insufficient for the formation of such prismatic loops, which accounts for the increase of stress in the multilayer.     

A model of size effects in nano-indentation

The indentation hardness-depth relation established by Nix and Gao [1998. Indentation size effects in crystalline materials: a law for strain gradient plasticity. J. Mech. Phys. Solids 46, 411-425] agrees well with the micro-indentation but not nano-indentation hardness data. We establish an analytic model for nano-indentation hardness based on the maximum allowable density of geometrically necessary dislocations. The model gives a simple relation between indentation hardness and depth, which degenerates to Nix and Gao [1998. Indentation size effects in crystalline materials: a law for strain gradient plasticity. J. Mech. Phys. Solids 46, 411-425] for micro-indentation. The model agrees well with both micro- and nano-indentation hardness data of MgO and iridium.     

纳米压痕残余应力场拉曼光谱实验研究

本文采用纳米压入仪在晶向为〈111〉和〈100〉的两种单晶硅片表面压入1000nm,卸载后得到深度约为550~570nm的压痕。使用共聚焦显微拉曼光谱仪对压痕周边区域进行测量,采用场扫描成像技术得到了压痕周边拉曼频移、半高宽、峰强等拉曼信息,通过分析由频移求得残余应力场的分布。在实验的基础上讨论了残余应力场的分布,以及晶向对应力场分布的影响,近似给出了压痕边缘最大压应力与微裂纹尖端最大拉应力。对其他拉曼信息的分析表明,半高宽和峰强信息与材料晶格结构的变化相关,在一定程度上也可以反映残余应力的作用。     

两种国产低活化的铁素体/马氏体钢的He离子辐照硬化研究

低活化的铁素体/马氏体钢（RAFM）以其高导热率、低热膨胀率、高抗辐照肿胀能力成为未来核聚变堆重要的候选结构材料,在聚变堆高能中子辐照环境由于（n,α）核反应产生的高浓度He在材料中的积累对于材料微观结构和宏观性能的影响是关系这类材料服役寿命的重要问题。本工作研究了面向聚变反应堆应用的两种国产低活化钢（CLF、CNS）的辐照硬化效应,利用中国科学院近代物理研究所320 kV高压实验平台提供的4He离子束进行辐照实验,辐照剂量6×10-3,6×10-2,6×10-1 dpa （辐照损伤/原子平均离位）,对应注He浓度分别为100,1 000,10 000 appm （氦离子浓度/百万分之一）。采用多能注入方法,在样品表面至1微米深度形成He浓度和离位损伤的坪区分布。利用纳米压痕仪对参比样品和注入He的样品进行了连续刚度测试。基于NIX-GAO模型对纳米硬度数据进行分析,获得了注入He的区域样品纳米硬度的数据。研究表明,注入He区域的纳米硬度与辐照损伤水平之间存在着1/2次幂函数的关系。未辐照CLF钢比CNS钢的纳米硬度略低,随着辐照剂量的增加,CLF钢呈现的辐照硬化现象更明显。Reduced activation ferritic/martensitic steels (RAFM) are important candidate materials for future fusion nuclear reactors because of their high thermal conductivity, low thermal expansion rate and high resistance to irradiation swelling performance. The influence of high concentration helium produced by nuclear reaction (n,α) on the micro-structure and macro-properties is an important issue limiting the service lifetime of the materials. In the present work, helium implantation to three different doses (100, 1 000, 10 000 appm helium, corresponding to 6×10-3, 6×10-2, 6×10-1 dpa) was carried out to investigate irradiation hardening of two RAFM Steels. Multi-energy He ion-beams at 320 kV high-voltage platform were used to get a damage plateau from surface to 1 μm depth in specimens. The continuous-stiffness test by a Nano-indentor G2000 was carried out Data of nano-hardness were analyzed based on Nix-Gao model. It is shown that there is a 1/2-power law relationship between the hardening and the irradiation damage level. Before helium implantation, the hardness of the CLF steel is slightly lower than that of the CNS steel. However, with the increase of helium-implantation dose, the hardening is more obvious in CLF steel. Further investigation of microstructures is needed to get a deeper understanding of the hardening mechanism.